This invention relates in general to pressure sensors used in hydraulic control systems and in particular to a high reliability pressure sensor utilized in a vehicle brake system with Hydraulic Brake Assist.
Recently, Hydraulic Brake Assist (HBA) has been included on new vehicles. HBA provides maximum braking capability during an emergency braking situation. During a braking cycle, the brake pressure is sensed to determine if an emergency situation has occurred. Alternately, the magnitude of the brake pedal stroke and speed of brake pedal movement can be monitored for an emergency braking situation. Typically, an emergency is identified by a certain pedal-application speed occurring along with a minimum level of brake-pedal force. Thus, a quick, deep stab at the brake pedal activates HBA while a quick shallow stab, as to cancel cruise control, or a slow but deep pedal application, as when slowing for a curve, will not activate HBA.
Upon detection of an emergency braking situation, HBA increases brake application pressure to a maximum value and continues to hold the maximum pressure until the vehicle stops or the brake pedal is released, as illustrated in FIG. 1. In FIG. 1, vehicle braking force is plotted as a function of time. The lower curve, which is labeled 4, is for a brake system without HBA, while the upper curve, which is labeled 6, is for a brake system that includes HBA. Typically, during an emergency braking situation, the vehicle operator partially lifts his foot from the brake pedal following his initial quick, deep stab. Thus, HBA assures that the brakes remain applied with maximum force.
There are a number of known methods for integrating HBA with a vehicle brake system. One method is completely mechanical and involves modification of the vacuum brake booster to provide HBA. Another method is to include the HBA function in an Anti-lock Brake System (ABS). An ABS is often included in many vehicles to prevent wheel lock up during stops upon low mu road surfaces. Such systems detect excessive slippage of one or more controlled wheels and selectively reduce and reapply the pressure applied to the controlled wheel brakes to reduce the slippage and thereby avoid a potential locking-up of the wheel.
Referring again to the drawings, there is illustrated in FIG. 2, a typical brake control system 10 which has HBA included in an Anti-lock Brake System (ABS). The brake control system 10 is intended to be exemplary and it will be appreciated that there are other brake control systems having different architecture than shown. In FIG. 2, a brake pedal 12 is mechanically coupled (not shown) to a brake light switch 13 and a dual reservoir master cylinder 14. The master cylinder 14 is connected to a hydraulic control unit 16 by a pair of hydraulic lines 18 and 20. The hydraulic control unit 16 includes a plurality of solenoid valves to control the brake pressure applied to the individual wheel brakes. The control unit 16 also typically includes a source of pressurized hydraulic fluid, such as a pump driven by an electric motor. The control unit 16 is connected via hydraulic lines 22, 24, 26 and 27 to individual wheel brakes (not shown) for the front wheels 28 and 30 and the rear wheels 32 and 33. Typically, the brake circuit is diagonally split with one master cylinder reservoir controlling the brakes associated with the left front wheel 30 and right rear wheel 33 and the other master cylinder reservoir controlling the brakes associated with the right front wheel 28 and the left rear wheel 32.
The brake control system 10 also includes a pair of front wheel speed sensors 34 that generate signals that are proportional to the speed of the front wheels 28 and 30 and a pair of rear wheel speed sensors 36 that generate signals that are proportional to the speed of the rear wheels 32 and 33. The wheel speed sensors 34 and 36 and the stop light switch 13 are electrically connected to an Electronic Control Unit (ECU) 38. The control unit 38 includes a microprocessor (not shown), that, under the control of an algorithm, selectively actuates the solenoid valves and pump in the control unit 16 to correct excessive wheel slippage.
The brake control system 10 further includes a pressure sensor 40 that monitors the hydraulic pressure in one of the master cylinder reservoirs. An pressure signal is supplied to the ECU 38. The microprocessor monitors the pressure signal and responsive thereto, upon detecting an emergency brake application, to actuate HBA.
A typical prior art pressure sensor assembly is illustrated generally at 44 in FIG. 3. The pressure sensor assembly includes a sensor element 46 that is electrically coupled to an Application Specific Integrated Circuit (ASIC) 47. Hydraulic pressure is applied to the sensor element 46. Both the sensor element 46 and the ASIC 47 are typically mounted in a common housing, that is shown schematically by the dashed line labeled 48 in FIG. 3. The sensor element 46 may include a plurality of strain gauges mounted upon one side of a thin diaphragm. The diaphragm is usually a disc formed from stainless steel. The strain gauges are typically arranged as a conventional half or full bridge circuit, such as, for example, a conventional thin film Wheatstone Bridge. The hydraulic brake fluid in the brake system is in contact with the side of the diaphragm opposite from the strain gauges. When the vehicle brakes are applied, the hydraulic brake fluid is pressurized and causes the diaphragm to deflect from its rest position. As the diaphragm is deflected by the applied pressure, the strain gauges are stretched or compressed, causing a change in the internal resistance of the gauges. The changed resistances result in a voltage appearing across the bridge circuit that is proportional to the magnitude of the pressure. The voltage is conditioned by the ASIC 47. The ASIC 47 generates an analog or digital pressure signal that is applied to an input port of an ECU microprocessor 49. The microprocessor 49 is included in the vehicle brake control system 10.